First of all, you might want to try words like "limit" instead of "cap".
mirc already has such a feature, but it applies to dcc sends that are initiated from a script. So the help file implies that it doesn't work if you manually /dcc send a file to somebody, but I don't know if this is actually true. Anyway, to read about this, type /help /dcc get and scroll down to "DCC Fileserver". Read the Note there.
